MOVING BACKWARDS IN FRONT STANCE (Zenkutsu dachi)
To eliminate this problem which is very common follow the steps below.
1. The first action is, instead of trying to step back straight away, transfer your body weight back onto the back leg. Practise this like a rocking motion back and forth but you must maintain good posture while you transfer.
2. If you are able to do this without lifting the heel then you should have no problem doing it at full speed once you have practised the shift.
3. Repat the backward step slowly on both sides of the body, over and over until it starts to feel smoother and the heel stays connected with the floor
Foot adherence to the floor is a common fault in competition and carries a deduction in your score when performing kata. It is also marked down in basic line work (kihon) when grading, so work on correcting this mistake and your basic techniques will be better.